Fuel cells are used as a power source for electric vehicles, stationary power supplies and other applications. One known fuel cell is the PEM (i.e., Proton Exchange Membrane) fuel cell that includes a so-called MEA (“membrane-electrode-assembly”) comprising a thin, solid polymer membrane-electrolyte having an anode on one face and a cathode on the opposite face. The MEA is sandwiched between a pair of electrically conductive contact elements which serve as current collectors for the anode and cathode, which may contain appropriate channels and openings therein for distributing the fuel cell's gaseous reactants (i.e., H2 and O2/air) over the surfaces of the respective anode and cathode.
PEM fuel cells comprise a plurality of the MEAs stacked together in electrical series while being separated one from the next by an impermeable, electrically conductive contact element known as a bipolar plate or current collector. In some types of fuel cells each bipolar plate is comprised of two separate plates that are attached together with a fluid passageway therebetween through which a coolant fluid flows to remove heat from both sides of the MEAs. In other types of fuel cells the bipolar plates include both single plates and attached together plates which are arranged in a repeating pattern with at least one surface of each MEA being cooled by a coolant fluid flowing through the plates.
The fuel cells are operated in a manner that maintains the MEAs in a humidified state. The level of humidity or hydration of the MEAs affects the performance of the fuel cell. Too wet of an MEA limits the performance of the fuel cell stack and may prevent high current density operation. Specifically, formation of liquid water impedes the diffusion of gas to the MEAs, thereby limiting their performance. The liquid water also acts as a flow blockage reducing cell flow and causing even higher fuel cell relative humidity which can lead to unstable fuel cell performance. Too dry of an MEA also limits the performance and may prevent high current density operation. Specifically, as the humidity level decreases the protonic conductivity of the MEA will start to increase (especially near the inlet), resulting in additional waste heat and lower production of electricity. Furthermore, durability data suggests that large cycling in the moisture content of the MEA that leads to flooded and dried membranes can lead to significant loss in durability due to repeated membrane swelling and shrinking. Thus, flooded and dry operating conditions limit high current density operation and may reduce the durability of the MEA and the fuel cell.
Accordingly, it is advantageous to control the operation of the fuel cell in a manner that prevents and/or minimizes flooded operation and/or dry operation of the fuel cell. Furthermore, it would be advantageous to control the operation of the fuel cell in a manner that results in high current density operation of the fuel cell thereby providing for efficient operation. Moreover, it would be advantageous if such operation were achievable over the nominal power operating levels of a fuel cell, including upward and downward transients in the power level.